A reduction in the emission of CO2, which is a greenhouse effect gas, is a global problem from the viewpoint of global environmental protection. For suppressing the emission from vehicles, the development of a fuel cell vehicle (FCV) using hydrogen as a fuel, which emits only water when burned, has progressed. In the FCV, it is desirable to mount a hydrogen concentration meter for hydrogen concentration control at the time of hydrogen combustion and sensing of hydrogen leakage from piping. Especially, hydrogen is known to explode when the concentration in the air reaches 3.9%; therefore, in the application for sensing of hydrogen leakage, a safety measure such as issuing an alert by the hydrogen concentration meter before the concentration reaches the above explosion limit concentration is desirable. Moreover, since the fuel economy performance can be improved by optimizing the hydrogen concentration at the time of combustion, it is desirable to monitor the hydrogen concentration for the purpose of providing feedback on combustion conditions.
Hydrogen sensors that are in practical use at present include those of a contact combustion type, a semiconductor type, and a gas heat conduction type. In addition to the above, types in the development stage include a solid electrolyte type and a FET type.
The contact combustion type is a type that detects the temperature rise of a catalytic metal such as platinum (Pt) or palladium (Pd) caused by the combustion of hydrogen on the surface of the catalytic metal. The contact combustion type has drawbacks such as having low selectivity while having high hydrogen sensitivity, requiring oxygen for detection, requiring a contrivance for a detection circuit because of a small output, and having large power consumption because the operating temperature is as high as approximately 400° C. and a device needs to be maintained at a high temperature.
The semiconductor type is such that a carrier concentration changes because, due to oxidation reaction of hydrogen on the surface of metal oxide semiconductor, a depletion layer within the semiconductor changes and a change in electrical conduction caused by the change in carrier concentration is detected. The semiconductor type has drawbacks such as requiring oxygen for causing oxidation reaction on the surface of metal oxide semiconductor, and having large power consumption because the operating temperature is as high as approximately 500° C. and a device needs to be maintained at a high temperature.
The gas heat conduction type is such that since a thermal conductivity changes with hydrogen being mixed into an atmosphere gas, hydrogen is detected by detecting the amount of change in the amount of heat released from a device. The gas heat conduction type has the advantage of small power consumption because the operating temperature is relatively as low as approximately 200° C., but is not suitable for use in the application for sensing of hydrogen leakage because it is difficult to detect hydrogen at a low concentration.
Moreover, as to the FET (field-effect transistor) type sensor, an example of having a Pt—Ti—O gate structure is disclosed in Non Patent Literature 1.